Kim was killed by a firing squad in July as "an anti-party, anti-revolutionary agitator", an official at the ministry, who declined to be named, according to the AFP news agency.

"Kim Yong-Jin was denounced for his bad sitting posture when he was sitting below the rostrum" during a session of North Korea's parliament, and then underwent an interrogation that revealed other "crimes", the official told reporters.

Unverified reports

The unification ministry said two other senior figures were forced to undergo re-education sessions.

One of them was Kim Yong-chol, a top official in charge of inter-Korean affairs and espionage activities against the South.

The 71-year-old Kim is a career military intelligence official who is believed to be the mastermind behind the North's frequent cyberattacks on Seoul.

Kim was banished to a farm in July for a month for his "arrogance" and "abuse of power", the ministry official said, according to AFP.

It is difficult to independently verify news about top officials in the North or the inner circle around the leader. Some previous reports of executions and purges in the reclusive state have proved inaccurate.

In February, the chief of the general staff, General Ri Yong-gil, was reportedly executed on charges of corruption, according to South Korea's Yonhap news agency.

In May, however, he reportedly reappeared on a list of attendees of a Workers' Party of Korea congress.

Kim Jong-un took power in 2011 after the death of his father, Kim Jong-il.